What would you need to do to convert? Well, Ghostwalk has a huge swath of rules content in it, items, spells, abilities, monsters - so anything that would generally need converting because its rules changed. Here's a link to general 3.0/3.5 changes you'd need to look out for. What are the major differences between D&D 3.0 and D&D 3.5? Determine what content you want to use out of Ghostwalk then run it by the conversion guide - and remember than specific monsters etc. do change in 3.5, compare vs. those books when it's something that exists in both editions.